Most common colour displays have a pixel structure with multiple colour components next to each other. Examples of such pixel structures are RGB stripes and the so-called “PenTile” arrangement. These arrangements require sub-pixellation, so that the native resolution of the display (i.e. the sub-pixels) is higher than the resolution of the image (i.e. the pixels).
An alternative is to employ emissive layers that are also transparent. A pixel is formed by multiple layers with each layer capable of emitting light in a selected colour. Organic light emitting diode (OLED) technology is one example that allows such a display to be formed. By stacking the transparent sub-pixel layers, the resolution of each sub-pixel layer only needs to be as high as the eventual pixel resolution of the display.
Reflective display techniques are also known, such as electrowetting displays and electrophoretic displays. These are in principle able to provide nearly full-gamut colour if combined in layers with a subtractive colour system such as cyan-magenta-yellow (CMY).
Limited colour displays are also known, which have only two colour components. An example is a subtractive colour system with black and red. Another example is an additive colour system with just blue and green.
The index of refraction (IOR) of optical materials such as glass and plastic is not a constant but rather depends on the wavelength due to dispersion. One way to describe the dispersion of a material is by the Abbe number. Dispersion in lenses creates chromatic aberration. The difference in focal length of red and blue is referred to as axial chromatic aberration. Chromatic aberration in the focal plane is lateral chromatic aberration. In photo camera lenses, techniques such as the use of an achromatic doublet are employed to avoid both types of chromatic aberration.
FIG. 1 shows an alternative type of lens based on diffraction. These lenses rely on the interference of light in a repetitive structure.
Diffractive components alter the phase and/or amplitude of light.
A Fresnel zone plate is shown in FIG. 1(a) and a fractal zone plate is shown in FIG. 1(b). These are flat structures, but they can focus light. A photon sieve, such as a fractal photon sieve as shown in FIG. 1(c) is a similar structure that has favourable properties.
The primary focal length of a zone plate (f) is given by Rm2/mλ with Rm being the radius of the mth ring. Thus, the focal length has a direct but inverse relation with the wavelength. For diffractive lenses, blue (e.g. 475 nm) has a longer focus than red (e.g. 650 nm) because of this relation. The width of the outmost zone (ΔRm) may be approximated by fλ/2Rm.
This zone width is an approximation that works for large m, so in this case Rm would typically be the outmost zone. The width of the zones decreases with radius, so the outmost zone determines the required manufacturing precision.
FIG. 2 shows the light intensity as a function of axial distance from a lenticular lens structure for red, green and blue light passing through the lens structure. FIG. 2(a) is for a Fresnel zone plate and FIG. 2(b) is for a Fresnel photon sieve. The intensity peaks correspond to the locations at which the light is focussed, and it can be seen that the different colours are focussed at different distances from the lens.
In a lenticular based autostereoscopic display, a lens overlies a group of sub-pixels in the row direction. In this way, the output of each sub-pixel in the group is imaged by the lens to a different viewing direction. At the intended viewing distance, adjacent pixels are imaged to locations spaced apart by the inter-ocular distance of around 60 mm, so that different eyes see a different sub-pixel set. In this way, autostereoscopic viewing is enabled, with different images provided simultaneously to the two eyes.
If a lenticular display is created from a thick stacked display, then the focus of the lens is optimal (i.e. with the focal plane at the display pixel plane) only for a small range of wavelengths—i.e. the wavelength corresponding to the stack colour which is at the focal plane. This may for instance be all saturated greenish colours. Other colours and white create 3D crosstalk.